Recurring dreams.
Because these dreams are presented to us more than once, we might assume that they are expressing an important message. We can look specifically for recurrences in our dream journal.
Themes often recur during the same sleep-period. When we are interpreting dreams from the same sleep-period, we can look for recurring themes; after interpreting one of the dreams, we can consider whether the same subject was examined in the others. This might aid us in the interpretation of those other dreams.
Recurring dreams are not necessarily identical. We might have dreams which contain a similar plot, characters, action, or feelings -- but the series might also differ in any of those elements (and we could even experience the same dream from the viewpoint of a different character). An important issue might be repeated verbatim, but it is more likely to be viewed with different symbolism, characters, plots, topics, and so on. This creates alternative perspectives from which the unconscious mind can study the topic.
Recurring dreams indicate an inability to find resolution. When the same dream occurs more than once, the unconscious mind is probably trying again to achieve a settlement following a previous unsuccessful attempt -- or it is presenting the same material to the conscious mind because we failed to follow through on that information during our wakeful life (perhaps because of inadequate recall or interpretation). If the psychological dilemma is alleviating but is not yet satisfied, we might see a change in the symbolism (i.e., "symbol evolution"); for example, the three-headed monster might now have only two heads!
We can create recurring dreams. If we are unable to interpret a dream, we can incubate a repetition of it; the dream might appear again, perhaps with symbolism which is easier to understand. And if we are in a lucid dream, we can create a scenario which is similar to the original dream; the plot is likely to go in a different direction, but we are still likely to gain insight into the original.
We can do "active imagination" with a recurring character. If a character continues to reappear, active imagination (as described in this book) might answer our questions about its presence. In a sense, this is a further recurrence of the dream, because we are creating that state during wakefulness. Several of my apparently unrelated dreams featured a teenaged boy, whom I "interviewed" with active imagination. He said, "I keep recurring [in your dreams] because I want you to know I'm here."